Ian,
The Technote says, "When predefining volumes, the Tivoli Storage Manager server 
will use all of the volumes in one directory before using the volumes in 
another directory."
Here is the example:
----------------
7) Sort list of empty volumes alphabetically: List appears as
c:\00000000.BFS
c:\00000002.BFS
c:\00000004.BFS
d:\00000001.BFS
d:\00000003.BFS
d:\00000005.BFS

8) Begin filling volumes in the order of the list from step 7. 
----------------

To the contrary, my experience is that predefined volumes are used in this 
order:
c:\00000000.BFS
d:\00000001.BFS
c:\00000002.BFS
d:\00000003.BFS
c:\00000004.BFS
d:\00000005.BFS

My filesystems are not on NFS, although I didn't see anything in the Technote 
stating it applies to NFS only.

Best wishes,
Keith